Termination-Accelerated Electrochemical Nitrogen Fixation on Single-Atom Catalysts Supported by MXenes

The synthesis of ammonia (NH3) from nitrogen (N2) under ambient conditions is of great significance but hindered by the lack of highly efficient catalysts. By performing first-principles calculations, we have investigated the feasibility for employing a transition metal (TM) atom, supported on Ti3C2T2 MXene with O/OH terminations, as a single-atom catalyst (SAC) for electrochemical nitrogen reduction. The potential catalytic performance of TM single atoms is evaluated by their adsorption behavior on the MXene, together with their ability to bind N2 and to desorb NH3 molecules. Of importance, the OH terminations on Ti3C2T2 MXene can effectively enhance the N2 adsorption and decrease the NH3 adsorption for single atoms. Based on proposed criteria for promising SACs, our calculations further demonstrate that the Ni/Ti3C2O0.19(OH)1.81 exhibits reasonable thermodynamics and kinetics toward electrochemical nitrogen reduction.

This work reported the transition metal atoms supported on MXene with O/OH terminations, as single atom catalyst (SAC) for electrochemical nitrogen reduction. They found that the OH terminations on Ti3C2T2 MXene can effectively enhance the N2 adsorption and decrease the NH3 adsorption for single atoms. This study would provide fast screening criteria for evaluation the catalytic performance of MXene-based SACs and a comprehensive understanding on effects of surface chemistry on their performance in the electrochemical processes of NRR. It is interesting and I suggest its acceptance after minor revision.

Response:
We thank the reviewer for the many suggestions which helped us improve our manuscript. MXenes. Furthermore, the electrochemical nitrogen reduction is considered on the Ir@v-Mo2CO2. The importance of the suggested paper has been emphasized in the manuscript (citation [34]). In our study, the screening of promising transition metals is based on the stability of anchoring transition metal atoms in the form of single-atom adsorption, the capability for metal atoms to capture N2, and the ability for transition metals to release the NH3. Taking into consideration that the electrochemical NRR can be proceeded along multiple pathways, we have investigated the adsorption behavior of two NH3 molecules in order to obtain a complete understanding for the catalytic performance of transition metals. Furthermore, we not only study the single transition metal atoms on the Ti3C2O2 MXenes, but also focus on the influence of the OH terminations on the catalytic performance. In this work, we have shown that OH terminations will not necessarily reduce the catalytic activity of the single atom. On the contrary, the existence of the OH terminations can effectively reduce the adsorption of NH3 on the single atoms, which would significantly accelerate the reaction kinetics. Based on the proposed criteria, we suggest that the Ni/Ti3C2T2 catalyst exhibits good catalytic activity and reasonable reaction kinetics towards electrochemical NRR. We believe that our study can provide a comprehensive understanding of the influence of the surface chemistry of MXenes on the catalytic performance of single metal atoms.
We appreciate the recommended literature from the reviewer. However, the suggested papers focus on the rational design of catalysts towards the nitrogen oxidation reaction [ACS Nano 2022, 16, 655] and the design of electrolyte in electrochemical NRR [Adv. Energy Mater. 2021, 11, 2101699], which will be of value for other studies. Therefore, we did not consider them relevant for the current work.
In the manuscript, we have added following discussion in the first section: In addition, studies focusing on the catalytic performance of single atoms supported on MXenes with a mixture of termination groups are limited in number. Therefore, it is necessary to take the OH terminations into consideration for both the stabilization of single metal atoms and their influence on the catalytic performance.
2. When calculation the change of Gibbs free energy for each elementary step at zero potential, the changes of the zero-point energy are usually applied. However, in supporting information, I didn't find this part in calculation section.

Response:
In this study, the contribution of the zero potential energy to the change of Gibbs free energy has been included in the vibrational enthalpy ( !"# ( )). For each elementary step of NRR, the reaction intermediates (NxHy) are chemisorbed to the transition metal atoms. The contribution to the Gibbs free energy from vibrational degrees of freedom is included by calculating the vibrational enthalpy !"# ( ) and the vibrational entropy !"# , as suggested by a previous study [Chem. Mater., 2021, 33, 9108]. The vibrational enthalpy is defined by (eq. S3): in which the first term is the zero-point energy (ZPE To the best of our knowledge, the fabrication of single sheets of Ti3C2S2 and Ti3C2Te2 has not been reported yet. Therefore, the application and the potential catalytic performance of such MXenes in SACs are beyond the scope of this study.
In the manuscript, the corresponding information has been added: Similar results can be observed for other terminations including Cl, Br, I, and S. Interestingly, the Ni/Ti3C2O0.19Te1.81 exhibits positive adsorption energies for the 2 nd NH3, suggesting promising reaction kinetics for electrochemical NRR. However, the synthesis of multilayers of the Te terminated Ti3C2 requires high temperature (300°C to 600°C) 49 and the fabrication of the Ti3C2Te2 monolayer has not been reported yet. Such obstacles would therefore hinder the further application of Te terminated Ti3C2 in single atom catalysis.  In order to understand the effects of other termination groups on the catalytic performance, the catalytic performance of single Ni atom supported on Ti3C2O0.19T1.81 (T = F, Cl, Br, I, S, and Te) MXenes are investigated based on proposed screening criteria. As listed in Table S2, Ni/Ti3C2O0.19S1.81 exhibits similar catalytic performance as the Ni/Ti3C2O2, in which strong NH3 adsorption would prohibit further catalytic cycles. In addition, halogen terminated Ti3C2 MXenes exhibit weaker Ni adsorption but can enhance the N2 adsorption comparing to the O terminated Ti3C2. Nevertheless, the potential catalytic performance for Ni/Ti3C2O0.19X1.81 (X = F, Cl, Br, and I) is limited by the strong NH3 adsorption. Interestingly, the Ni/Ti3C2O0.19Te1.81 exhibits positive adsorption energies for the 2 nd NH3 (0.21 eV), suggesting promising reaction kinetics for electrochemical NRR. However, practical challenges for multilayers of Te terminated Ti3C2 MXenes (Ti3C2Te2) remain at the harsh synthetic conditions (300°C to 600°C in molten alkali metal halides). 14 Furthermore, the fabrication of the Ti3C2Te2 monolayer has not been reported yet, which prevents further applications in single atom catalysis.

Response:
We thank the reviewer for the valuable suggestion. We have added more discussion regarding to the reaction pathway and the energy profiles of the electrochemical NRR on Ni/Ti3C2O2 and Ni/Ti3C2T2.
The following discussion has been added into the manuscript: Moreover, the second hydrogenation step along the enzymatic pathway exhibits an exothermic characteristic on the Ni/Ti3C2T2, while a Gibbs free energy barrier of 0.14 eV should be overcome on the Ni/Ti3C2O2 (Figure 4b). Such discrepancy in Gibbs free energy profiles indicates that the single Ni atom exhibits different selectivity on the Ti3C2T2 and Ti3C2O2. As a result, the enzymatic pathway is no longer energetically favored while the alternating pathway exhibits the lowest overall Gibbs free energy of 1.05 eV on the Ni/Ti3C2O2. Despite, the catalytic performance of the Ni/Ti3C2O2 catalyst is profoundly limited by the strong interactions between the single Ni atom and NH3 molecules (G(NH3) = -1.60 eV and G(N2H6) = -1.51 eV).

Reviewer 2:
Niu et al. present a systematic computational search for single-atom catalysts for nitrogen reduction supported on MXene substrates. Specifically, they compare adsorption energies of single metal atoms vs dimers as a proxy for obtaining well-dispersed catalyst atoms on the substrates, and those of nitrogen vs ammonia to identify candidates that strongly bind the reactant but can release the product. Based on these metrics, they identify Ni supported on the Ti3C2T2 MXene as the best candidate and map out the energetics of the nitrogen reduction reaction on this material in detail. Overall, the study is well-organized, systematically scanning the complex space of materials to find a promising catalyst, and is well-written, meriting publication in JPCL.

Response:
We thank the reviewer for the many suggestions which helped us strengthening our manuscript.
It would strengthen the manuscript to further justify the choice of adsorption energies as a metric for the catalyst stability. In particular, all computed energies are for neutral adsorbates in vacuum. This could change substantially in the electrochemical environment, especially given the potentially harsh potentials required for nitrogen reduction.  (Figure 1). Such enhanced adsorption of metal atoms indicates that the TM/Ti3C2O2 catalysts exhibit higher stability with implicit solvation. Of importance, the stability of the single atom adsorption on the Ti3C2O2 is further assessed by considering corrosion reactions of TM/Ti3C2O2, in which the corrosion potentials for each transition metal atoms are calculated (Table S1 and eq S9-S14). As a result, most transition metals can be stablized in the form of single atoms on the Ti3C2O2 when the applied potential is smaller than -1 V vs. SHE. Considering that the electrochemical NRR takes place at the cathode and the negative potentials are required, the single atom dispersion of transition metal atoms can thus be guaranteed (detailed discussion in the response to reviewer 3 below).
Despite, the ability for single atoms to bind the N2 molecule is decreased based on the implicit solvation model. As a result, the N2 will not be stablized on the transition metal atoms such as Sc, Ti, Zr, Nb, and Mo supported on the Ti3C2O2 (Figure S2). However, limited suppression on the NH3 adsorption has been observed with implicit solvation model (Figure S3), in which the NH3 maintains stronger interactions with single transition metal atoms than that of N2 in solution. In addition, further calculations have revealed that introducing OH terminations to the Ti3C2 MXene is still an effective approach to promote the catalytic performance in solution. The updated adsorption energies for the N2 and NH3 on TM/Ti3C2O2 and TM/Ti3C2T2 are summarized in Figure 2. As shown in Figure 2b, the adsorption energy of the NH3 on the TM/Ti3C2T2 MXene is significantly decreased. For metal atoms including Fe, Co, V, and Ni, the adsorption of NH3 is energetically less favored by more than 1 eV. Specifically, the NH3 exhibits positive adsorption energies on Ag, Nb, and Mo catalysts, indicating a spontaneous desorption. Furthermore, the capability of single atoms to capture N2 is strengthened on the Ti3C2T2 support. As shown in Figure 2c-2d, the N2 tends to bind stronger on single atoms supported on Ti3C2T2. For example, the N2 exhibits positive adsorption energies on Nb, Mo, Ti, and Zr atoms supported on the Ti3C2O2, while such adsorption is significantly promoted when metal atoms are anchored on the Ti3C2T2. Despite that Nb and Mo supported on the Ti3C2T2 possess ideal N2 and NH3 adsorption, Nb and Mo SACs may exhibit unstable catalytic performance as the N2 adsorption on Nb and Mo is highly sensitive to OH terminations. Furthermore, the co-adsorption of two NH3 molecules on single atoms with implicit solvation model is investigated. As shown in Figure 3, the 2 nd NH3 exhibits stronger than that of the 1 st NH3 under solution condition for most of single atoms. Consequently, the capture of the N2 and further hydrogenation steps would be hindered. Of importance, only Ni exhibits positive adsorption towards the 2 nd NH3 molecule, suggesting that the single Ni atom can exhibit fast kinetics, which is beneficial to further reduction reactions. To summarize, the solvent effect can enhance the single atom adsorption of TM on the Ti3C2O2 and decrease the adsorption of NH3 on the TM/Ti3C2T2, which would promote the catalytic performance. Finally, the solvation effect on the Gibbs free energy profile of NRR on Ni/Ti3C2T2 has been clarified. As shown in Figure S4, both the highest relative Gibbs free energy and the limiting free energy barrier have been reduced after including the solvation effect. To be specific, the limiting barrier decreases from 1.56 eV (vacuum) to 1.36 eV (implicit solvation), indicating a promoted catalytic activity of Ni/Ti3C2T2. Besides, the relative Gibbs free energy of NH3+NH3 species (N2H6 in Figure S4) increases to -0.04 eV, which would accelerate reaction kinetics.
We have added more discussion and updated figures in the manuscript: Herein, the adsorption behavior of transition metal atoms is investigated under both vacuum and implicit solvent conditions.  Notably, the NH3 possesses positive adsorption energies on Ag, Nb, and Mo single atoms, indicating a spontaneous desorption of NH3.
As seen in Figure 2c and 2d, the adsorption N2 is significantly strengthened on Mo, Nb, Cr, and Ti supported on the Ti3C2T2, while N2 exhibits positive adsorption energy on the same single atoms supported on the Ti3C2O2.
To this end, single atoms (Fe, Co, Ni, V, Nb, and Mo) supported on the Ti3C2T2 MXene can be considered as the promising catalysts towards electrochemical NRR due to the desired adsorption behavior of both N2 and NH3. However, the capability to bind N2 for V, Mo, and Nb single atoms is highly sensitive to the amount of OH terminations on the MXene, resulting in unstable catalytic performance. In the manuscript, following information has been added: Furthermore, our calculations show that the Ni/Ti3C2T2 exhibits better catalytic activity under implicit solvation condition. As seen in Figure S4, the limiting step for the Ni/Ti3C2T2 along the enzymatic pathway is decreased from 1.56 eV to 1.36 eV. In addition, the relative Gibbs free energy of *NHNH is reduced to -0.11 eV in solution, indicating a more stable intermediate. Of importance, the accelerated reaction kinetics can be expected in the solution because the relative Gibbs free energy of N2H6 has been increased from -0.28 eV (in vacuum) to -0.04 eV (in implicit solvation).
In supporting information, we have added the following section: The Gibbs free energy profile for enzymatic pathway. Figure S4. The Gibbs free energy profile for the electrochemical NRR on Ni/Ti3C2T2 along the enzymatic pathway in vacuum (purple) and with implicit solvation (blue). Figure S4 shows the influence of the implicit solvation effect on the catalytic performance of Ni active sites. As seen, the N2 adsorption is more stable in the solution than in vacuum. The limiting Gibbs free energy barrier has been decreased from 1.56 eV to 1.36 eV, indicating the catalytic performance of the Ni atom is promoted in the solvent condition. In addition, reaction intermediate states such as *NNH and *NHNH bind stronger to the Ni in the solution, leading to a promotion of reaction kinetics. Furthermore, the implicit solvation effect exhibits positive effect on the adsorption of NH3, in which the Gibbs free energy of co-adsorption of two NH3 molecules (N2H6) is decreased from -0.28 eV to -0.04 eV. Such weak adsorption of NH3 indicates the Ni active sites can be available rapidly, resulting in accelerated reaction kinetics.
Similarly, the reaction analysis is based on proton-coupled electron transfer steps, which could be a poor approximation for several of the nitrogen reduction intermediates. Qualifying the results shown in the manuscript to indicate these potentially important effects would be useful. Corresponding information has been added into the Supporting information:

Response
The computational hydrogen electrode (CHE) method proposed by Nørskov et al is employed to calculate Gibbs free energy profiles for electrochemical NRR, 5 in which the electrochemical nitrogen reduction reaction is: , ( ) + 6 -+ 6 . = 2 / , including 6 proton-electron pair (H + + e -) transfer steps.
According to the CHE model, the chemical potential of the H + /epair under standard conditions is equal to the half of the Gibbs free energy of the H2 (pH = 0, p = 1 bar, T = 298.15 K). The Gibbs free energy of H2 is defined by: where 0 # 1213 , 0 # 45678 ( ) , 0 # 594 ( ) , and 0 # !"# ( ) are the electronic, translational, vibrational, and rotational enthalpies of H2, respectively. The translational enthalpy is defined by: the rotational enthalpy for H2 is defined by: the vibrational enthalpy and entropy are defined the same as eq. S3. The tabulated value of entropy of the H2 ( 0 # ) from as used. 7

Reviewer 3:
The authors investigated the potential of a single transition metal atom anchored on the Ti3C2T2 (T = O and/or OH) MXene as electrocatalyst for NRR and the effect OH terminations on the catalytic performance. While a key point is that the adsorption energy of single transition metal Ni on the Ti3C2O2 MXene is positive, see fig.1 and formula (1), that means Ni is not adsorbed on the surface of Ti3C2O2, based on this positive adsorption energy, we do not have any next step for NRR or other electrocatalysis performances on Ni@ Ti3C2O2 system. This paper is not recommended because it does not provide correct physical insights.

Response:
We thank the reviewer for the comment.
In our work, the adsorption energy of metal atoms is employed as a descriptor to characterize the possibility of agglomeration, in which metals in the gray region in Figure 1 may form cluster on the MXenes.
For an electrochemical reaction, it is necessary to evaluate the stability of single atom catalysts in solution. Herein, we performed the single atom adsorption on the Ti3C2O2 with implicit solvent model and calculated required potential for stabilizing single atoms. To demonstrate the stability of single metal atoms, we have considered the corrosion of single metal atoms. Corresponding reactions are defined by: ( ) → 7-+ . in which M refers to the metal atom, and n refers to the number of electrons the cation loses after dissolution. Specifically, for the Nb and Mo, the corrosion reactions are defined as: .
The corrosion potentials refer to the maximum potential for transition metals to maintain their singleatom adsorption. As listed in Table S1, the corrosion potentials for most 3d and 4d metals are in the range from -1.0 to 0.5 V. In addition, the electrochemical NRR not only take places at the cathode where the negative potential is applied but also requires negative potential to procced hydrogenation steps. Specifically, the corrosion potential for Ni supported on Ti3C2O2 in solution is -0.941 V, which is larger than the potential to proceed the NRR on Ni active site (-1.36 eV, seen from Figure S4). Therefore, it is reasonable to deduce that transition metal atoms will maintain the single atom dispersion in the electrochemical environment for nitrogen reduction reactions. In addition, the effect of the implicit solvation model on the adsorption of the single metal atoms, N2, and NH3 has been elaborated in the response to the review 2 (Figure 1-3, Figure S1-S4), in which the single Ni atom can be stablized on the Ti3C2T2 MXene and exhibits good catalytic performance towards electrochemical NRR.
In the manuscript, we have added information as follows: In addition, the stability for the single atom adsorption is assessed by the corrosion potential of TM/Ti3C2O2 in solution environment. The corrosion reactions of transition metals are the dissolution of transition metals to form the most stable cations/anions (eq. S9-S11) and corrosion potentials (calculated by eq. S12-S14) at pH = 0 are the highest potentials to stabilize the single atom adsorption (Table S1). It is found that corrosion potentials for the majority of single transition metal atoms on the Ti3C2O2 are in the range from -1.0 V to 0.2 V (vs. SHE). Taking into account that the electrochemical NRR requires negative potentials, the corrosion can be effectively prevented during the NRR process.
Corresponding section has been added into the supporting information: